pgsql: Rework EXPLAIN for planner's buffer usage.

Rework EXPLAIN for planner's buffer usage.

Commit ce77abe63c allowed EXPLAIN (BUFFERS) to report the information
on buffer usage during planning phase. However three issues were
reported regarding this feature.

(1) Previously, EXPLAIN option BUFFERS required ANALYZE. So the query
had to be actually executed by specifying ANALYZE even when we
want to see only the planner's buffer usage. This was inconvenient
especially when the query was write one like DELETE.

(2) EXPLAIN included the planner's buffer usage in summary
information. So SUMMARY option had to be enabled to report that.
Also this format was confusing.

(3) The output structure for planning information was not consistent
between TEXT format and the others. For example, "Planning" tag
was output in JSON format, but not in TEXT format.

For (1), this commit allows us to perform EXPLAIN (BUFFERS) without
ANALYZE to report the planner's buffer usage.

For (2), this commit changed EXPLAIN output so that the planner's
buffer usage is reported before summary information.

For (3), this commit made the output structure for planning
information more consistent between the formats.

Back-patch to v13 where the planner's buffer usage was allowed to
be reported in EXPLAIN.

Reported-by: Pierre Giraud, David Rowley
Author: Fujii Masao
Reviewed-by: David Rowley, Julien Rouhaud, Pierre Giraud
Discussion: https://postgr.es/m/07b226e6-fa49-687f-b110-b7c37572f69e@dalibo.com

Branch
------
REL_13_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/674899ae02c375b03411c0676e7cfb4bafeebac9

Modified Files
--------------
doc/src/sgml/ref/explain.sgml | 3 +-
src/backend/commands/explain.c | 46 ++++++++++++++---------------
src/test/regress/expected/explain.out | 55 ++++++++++++++++++++++++++++++++---
src/test/regress/sql/explain.sql | 2 ++
4 files changed, 77 insertions(+), 29 deletions(-)